The closest prior art presently known to the applicant includes E. L. Schlage U.S. Pat. Nos. 2,803,959 of Aug. 27, 1957, 3,257,831 of June 28, 1966 and 3,575,024 of Apr. 13, 1971. Of interest may be Tornoe U.S. Pat. No. 2,738,666 of Mar. 20, 1956 and Neary U.S. Pat. No. 3,824,819 of July 23, 1974.
While deadbolts have been mounted on hinged door panels to interconnect them with the door frame for a long time, there has recently been an increasing tendency to use deadbolts operable by keys from both sides in order to afford the highest security. In instances wherein the inside of the door is the sole exit from a room, there is danger in that the user may withdraw the key from the lock after having locked himself in. This may be satisfactory from the standpoint of security, but it can be dangerous under fire or other panic conditions as the user may misplace the key or in a panic be unable to retrieve the key for use.
It is therefore an object of the invention to provide a key retaining cylinder for a lock effective to keep the key in the lock whenever the lock bolt is extended or in locked position, particularly in a cylinder installed on the interior side of a door panel.
Another object of the invention is to provide a key retaining cylinder for a lock in which the operating key cannot be removed from the cylinder except when the bolt controlled by the lock is in its retracted or open position.
Another object of the invention is to provide a key retaining cylinder for a lock in which substantially standard lock parts are utilized for most of the structure, the retaining feature being incorporated in a simple, effective, economical fashion.
Another object of the invention is to provide a key retaining cylinder for a lock which additionally may have a shut-out feature.
A further object of the invention is to provide a key retaining cylinder for a lock which does not differ substantially in appearance or in regular operation from locks that are virtually standard and are well recognized.